Current production of printed circuit boards for the insertion of SMDs takes place, by way of example, through a HAL tin plating. However, severe fluctuations in the effective coating depth occur with this method, and for larger pads, overly light coating levels are frequently ascertained across considerable portions of the pad surfaces with all the negative consequences for subsequent working properties. One such consequence is that the individual connecting legs of the SMD components to be soldered on, ICs, for example, are not soldered.
The so-called optipad printed circuit board procedure is known for eliminating disadvantages of this type; this requires four individual procedural steps. In the first step, application of a temporary solder stop mask with a vacuum laminator takes place over the entire surface of both sides of the printed circuit board. Afterwards, the temporary solder mask is deposited, exposed and developed in a second step. A "cup structure" emerges as a result with precisely defined cavities in the area of the pads to be formed. This cup structure is filled with amounts of solder corresponding to the volume of each of the desired cavities. The solder sets afterward with a flat surface. The temporary solder stop mask is then removed in an aligning step. This procedure is very expensive due to the procedural runoff, which makes an additional mask necessary. In addition, there is a question here of a procedure which requires procedure specific facilities previously not used in the preparation of printed circuit boards.
Also known is the so-called SIPAD procedure, whereby a solder stop foil serving as a pad mask is used as a solder stop resist, and the recesses of the mask filled with solid solder by means of remelting with paste solder such that the surface is aligned level with the solder stop foil. The depth of the solder stop foil and of its recess sets the solder amounts in the solder locations. The SMDs are placed on the solder deposits and then heated to over the soldering temperature. The flat solder deposits deform into round solder bulges in the melting-on process which then rise above the solder stop foil. The level of rise depends, above all, on the geometry of the solder deposit and the soldering atmosphere. After the remelting and eventual cleaning away of the remaining flux, the initially bulge-shaped solder is pressed flat. Likewise, disadvantageous with this procedure there is a very expensive procedural runoff; the printed circuit boards are also exposed to high temperatures a number of times.